Monodispersed sprays of droplets of micrometric size have attracted the interest of scientist and engineers because of their potential applications in many fields of science and technology. Recently, the possibility of getting medicines into patients via pulmonary inhalation is being actively investigated by pharmaceutical companies around the world R. F. Service (1997), "Drug Delivery Takes a Deep Breath," Science 277:1199-1200. Classifying a polydispersed aerosol (for example, by using a differential mobility analyzer, B. Y. Liu et al. (1974), "A Submicron Standard and the Primary Absolute Calibration of the Condensation Nuclei Counter," J. Coloid Interface Sci. 47:155-171 or breakup process of Rayleigh's type of a capillary microjet Lord Rayleigh (1879), "On the instability of Jets," Proc. London Math. Soc. 10:4-13, are the current methods to produce the monodispersed aerosols of micrometric droplets needed for such applications. The substantial loss of the aerosol sample during the classification process can severely limit the use of this technique for some applications. On the other hand, although in the capillary break up the size distribution of the droplets can be very narrow, the diameter of the droplets is determined by the jet diameter (approximately twice the jet diameter). Therefore, the generation and control of capillary microjets are essential to the production of sprays of micrometric droplets with very narrow size distribution.
Capillary microjets with diameters ranging from tens of nanometers to hundred of micrometers are successfully generated by employing high electrical fields (several kV) to form the well-known cone-jet electrospray. Theoretical and experimental results and numerical calculations on electrosprays can be obtained from M. Cloupean et al. (1989), "Electrostatic Spraying of Liquids in Cone Jet Mode," J. Electrostat 22:135-159, Fernandez de la Mora et al. (1994), "The Current Transmitted through an Electrified Conical Meniscus," J. Fluid Mech. 260: 155-184 and Loscertales (1994), A. M. Ganan-Calvo et al. (1997), "Current and Droplet Size in the Electrospraying of Liquids: Scaling Laws," J. Aerosol Sci. 28:249-275, Hartman et al. (1997), "Electrohydrodynamic Atomization in the Cone-Jet Mode," Paper presented at the ESF Workshop on Electrospray, Sevilla, Feb. 28-Mar. 1, 1997 among others [see also the papers contained in the Special Issue for Electrosprays (1994)]. In the electrospray technique the liquid to be atomized is slowly injected through a capillary electrified needle. For a certain range of values of the applied voltage and flow rate an almost conical meniscus is formed at the needle's exit from whose vertex a very thin, charged jet is issued. The jet breaks up into a fine aerosol of high charged droplets characterized by a very narrow droplet size distribution. Alternatively, the use of purely mechanical means to produce capillary microjets is limited in most of applications for several reasons: the high-pressure values required to inject a liquid through a very narrow tube (typical diameters of the order of few micrometers) and the easy clogging of such narrow tubes due to impurities in the liquid.
The present invention provides a new technique for generating steady microcapillary jets exclusively based on mechanical means which does not present the above inconveniences and can compete advantageously with electrospray atomizers. The jet diameters produced with this technique can be easily controlled and range from below one micrometer to several tens of micrometers.